Studies of alpha-fetoprotein (AFP) and human chorionic gonadotropin (HGC) quantitated by radioimmunoassay (RIA) have been continued in clinical collaborations to aid in the diagnosis and monitoring of patients with hepatocellular carcinoma (HC) and germ cell tumors of the testis. Animal models for HC are available in monkeys and mice and are being studied for correlations of tumor size with serum AFP levels during tumor development and during various types of therapy. Development of a more sensitive RIA for AFP has allowed more accurate evaluation of serum AFP under normal conditions as well as in non-neoplastic liver disease. A RIA for a a new human lung tumor-associated antigen (LTAA) has been developed and used to demonstrate serum levels in lung cancer higher than those of normal controls. Studies are in progress to correlate the LTAA with stage of disease and evaluate specificity for differentiating lung cancer from benign lung disease and other types of tumors. A cross-reactive, but different protein has been discovered in normal human serum; isolation and identification of this antigen should lead to increased specificity of the RIA for LTAA. A non-virion surface antigen has been identified on a Moloney virus-induced lymphoma (MBL-2) and efforts to isolate and characterize this new antigen are in progress.